1. Field of the Invention
Encapsulation is rapidly becoming a major technology for formulating bioactive agents. Encapsulation has significantly extended field life of agricultural pesticides by offering protection from environmental exposure and resultant chemical and biological degradation. Moreover, it has allowed application of many pesticides at reduced dosages and less frequent intervals, thus reducing environmental contamination and extending residual activity.
Encapsulation technology similar to that developed for chemical pesticides could be useful to protect entomopathogens such as Bacillus thuringiensis (B.T.), nuclear polyhedrosis viruses, microsporidians, and other biocontrol agents. Most biocontrol agents are susceptible to rapid environmental degradation caused by exposure to ultraviolet radiation, heat, desiccation, substrate pH, and microbial competition, which severely limits their practical utility.
Economically feasible formulation technology that provides long-term protection of biocontrol agents from environmental degradation and also promotes infection of the target pest is critically needed to further their use in applied pest control. This invention relates to a novel encapsulation system which satisfies these criteria.
2. Description of the Prior Art
Techniques, including the use of clay granules [E. S. Raun et al., J. Econ. Entomol. 59: 620-622 (1966); S. M. Ahmed et al., Pestic. Sci. 4 19-23 (1973)], UV-absorbing compounds [R. P. Jaques, Can. Entomol. 104: 1985-1994 (1972); D. L. Hostetter et al., J. Kansas Entomol. Soc. 48: 189-193 (1975)] and encapsulation, have been developed for short-term environmental protection of entomopathogens. In the latter case, microencapsulation of nuclear polyhedrosis viruses with polyvinyl alcohol, ethylcellulose, or other polymers, coupled with UV-screening agents has been achieved [C. M. Ignoffo et al., J. Econ. Entomol. 64: 850-853 (1971); D. L. Bull et al., J. Econ. Entomol. 69: 731-736 (1976)]. However, field studies with these formulations produced variable results in the degree of pathogen survival and efficacy against the target insect. Calcium alginate used to encapsulate steinernematid and heterorhabditid nematodes offers promise, although the ability of the alginate to maintain moisture conditions conducive to nematode survival was a problem [H. K. Kaya et al., Environ. Entomol. 14: 572-574 (1985)].
The use of starch has many attractive properties for biocontrol agent encapsulation. First, it is inert and will not alter resting stages of most living organisms; second, particulate or liquid UV-screening agents are easily added; third, its major component is amylopectin which is readily digested by most phytophagous pests possessing .alpha.-amylase enzymes [G. M. Chippendale et al., J. Insect Physiol. 20: 751-759 (1974); K. Nishide et al., J. Fac. Agric. Tottori Univ. 11: 12-22 (1976)]; and fourth, it is abundant and inexpensive compared to most other materials currently used in encapsulation [B. S. Shasha, In Controlled Release Technologies: Methods, Theory, and Applications, Vol. 2, A. F. Kydoniens (ed.), CRC Press, Inc., Boca Raton, FL].
Recently, pesticides have been encapsulated in starch, crosslinked with borate, calcium, or xanthide, thereby producing a matrix that can be processed into granules of desired sizes, densities, and porosity [B. S. Shasha et al., J. Appl. Polym. Sci. 29: 67-73 (1984); D. Trimnell et al., J. Polym. Sci. 27: 3919-3928 (1982); R. E. Wing et al., J. Polym. Sci. 21: 121-140 (1983)]. Unfortunately, this encapsulating process is not suited for most biocontrol agents because the reagents and conditions of the crosslinking process are too harsh for their survival.
Controlled release by means of starch-based encapsulating materials can also be accomplished without the use of chemical crosslinking reactions. In U.S. Patent No. 2,876,160, Schoch et al. disclose such a method which employs modified, amylose-free starches at concentrations up to 65% solids for embedding water-insoluble materials.
In PCT Int. Appl. WO 85/04074, Flashinski et al. disclose two methods of preparing a starch gel matrix containing an insecticide. The insecticide is either coextruded with a dilute, aqueous dispersion of starch, or the starch is first partially cooked in an extruder prior to cold-blending with the insecticide. In either case, the product is recovered and used as an aqueous gel.
In U.S. Pat. No. 4.230,687, Sair et al. disclose the application of shearing stress, vigorous mechanical working, and heat to distribute active agent into an enveloping matrix of chemically modified starches, gums, and proteins in the presence of a limited quantity of water. Proteins are used for slow-release matrices; modified starches are used for rapid release.
Similarly, in U.S. Pat. No. 3,922,354, Galuzzi et al. disclose the use of high-shear mixing to incorporate active agents into low-water, high-solids matrices prepared from partially gelatinized unmodified starches. Additives such as modified dextrins, mixtures of mono- and diglycerides, toasted cereal solids, and coloring agents are used to control the release of active agents.
In U.S. Pat. No. 3,666,557, Jensen et al. disclose a method of using low-fat starchy materials to microencapsulate individual beadlets of sensitive materials such as vitamins and vegetable oils. Starches are prepared for encapsulation by heating at 88.degree. C. for 30 min followed by passage through a homogenizer to effect disruption of granules without degradation of molecules.